In the United States, tensile testing is generally performed in accordance with the procedures defined in ASTM E8 “Standard Test Methods for Tension Testing of Metallic Materials” and ASTM A370 “Standard Test Methods for Tension Testing of Steel Products”. Under those test methods,
Percent Elongation=((Final gage length minus Original gage length) divided by Original gage length) times 100,
where Original gage length is the length between gage markings on the tensile specimen before testing and Final gage length is the length between the same gage markings after testing.
The standard tensile specimen diameter is nominally 0.5 inch. A 2-inch gage length is generally used for round, solid or tubular standard size tensile specimens. Gage length is generally understood to mean the length of the marked region on the tensile specimen over which elongation is calculated. As used herein, the term gage length will refer to the original gage length unless stated otherwise. When smaller diameter solid or tubular specimens are tensile tested to determine the material's mechanical properties, the elongation measured using the standard 2-inch gage length decreases with decreasing specimen diameter. This result is termed the “specimen size effect” on elongation. The specimen size effect is observed even when the large and small diameter specimens are taken from the same bar of raw material. There are several potential causes that need to be considered for the specimen size effect on elongation.
One potential cause is the relative size of defects present on the surface of a specimen. For ductile materials that are not notch sensitive, isolated notches less than about 5% of the specimen section (diameter for solid specimens, wall thickness for tubular specimens), do not produce an appreciable reduction in observed tensile elongation. However, for larger defects, representing a significant portion of the specimen section (i.e., greater than about 5%) localized strain (strain is generally understood to be an expression of deformation caused by the action of stress on a physical body) is increased at the defects. The result is premature fracture at the defects and lower measured tensile elongation of the overall specimen. On specimens with sections larger than about 0.010 inch, defects representing 5% or more of the specimen section can be controlled by careful sample preparation.
A second potential cause is the relation between grain size and the specimen section. As the specimen section approaches the material grain size, dislocation motion is not readily accommodated by grains with less favorable crystallographic orientations. The results are premature fracturing at grain boundaries (i.e., the zone formed at the junction of individual crystals in a polycrystalline material) of the unfavorably oriented grains and low measured elongation. The premature fracturing can be significantly improved by preparing a specimen having at least three different grains across the specimen section. For the fine grained microstructures required for small medical devices, this effect is not significant for section sizes greater than about 0.010 inch.
A third potential cause is the decrease in the proportion of necking elongation to the total elongation to failure as the specimen diameter is reduced. This effect is believed to be the predominant cause of low measured elongation for solid or tubular tensile specimen with diameters from about ⅜ inch down to about 0.020 inch when measured with a constant gage length.
The current approach to reduce the specimen size effect on measured elongation is to use a proportional gage length that varies with the specimen diameter or area. ASTM specifications allow a proportional gage length of 4 times the specimen diameter for round specimens less than 0.5 inch diameter. ISO standards specify a proportional gage length of 5.65 times the square root of the specimen cross sectional area for small diameter specimens. This is equivalent to 5 times the diameter for round, solid specimens. These proportional gage length correction factors are intended to maintain the ratio between diameter and gage length found on the standard 0.5 inch diameter specimen with a 2 inch gage length on specimens with smaller diameter. They produce an elongation value similar to the standard 0.5 inch specimen, but are difficult to implement in the testing laboratory due to the non-standard gage lengths that are required.
In addition, because of the many dimensional and proportional gage lengths recommended by industry, national and international specifications, the various measured elongations for a particular alloy and condition cannot be readily compared to each other or evaluated versus specification requirements written for a different gage length. Tables have been created by ASTM and ISO to allow conversion of elongation values for specific materials and specific gage lengths. These tables are limited to the most common alloys and gage lengths.
If the measured elongation for a given material varies with the choice of gage length, the resulting elongation values cannot be claimed to be a material property. This has not been a major issue historically because the great majority of engineered structures have been designed to withstand stresses no greater than the material's yield strength. As a result, device performance has been determined predominately by the material's elastic properties and elongation has been used only as a relative measure of the material's resistance to catastrophic failure.
Recently, medical devices called arterial stents have been designed to deform plastically during balloon expansion and take a permanent set once expanded within the artery. During the design process, the material's plastic properties, including elongation, are required to determine at what point the stent will fail in the plastic strain environment experienced during balloon expansion.
Tubing used to fabricate tubular stents is smaller than 0.250 inch and typically smaller than 0.125 inch diameter, considerably less than the 0.5 inch standard specimen diameter. This tubing is tensile tested at its finished size and material condition to determine the arterial stent material properties.
The problem currently facing device designers who wish to FEA model the plastic stain behavior of a device such as an arterial stent is the uncertainty as to which elongation value (derived from various gage lengths) should be used as a material property to accurately model the device behavior. Once the choice is made, there is still uncertainty regarding how the elongation value should be used since it includes two undifferentiated components (uniform and necking elongation) which have significantly different implications concerning the resistance of the device to tensile failure.
Elongation differentiated into its uniform and necking components, may allow optimization by the material manufacturer of one or the other component to improve device performance.
As a result, there is a need for a method of measuring and calculating elongation on small diameter solid or tubular specimens that that is unaffected by the specimen size effect, that uses a universally applicable gage length marking technique, that produces elongation values at any gage length to allow comparisons with published data and specifications using alternate gage lengths, and that differentiates between uniform and necking elongation. For continuity with current published data and specifications, the method should be capable of producing elongation values similar to those obtained on the standard 0.5 inch diameter specimen at the 2 inch gage length. From the method, an elongation value could be calculated that is representative of the tested material's plastic deformation properties independent of gage length and specimen size.